1. Field of the Invention
The present invention relates to a positive electrode material having high capacity and high safety for a lithium ion secondary battery, and to a lithium ion secondary battery.
2. Description of the Related Art
For the application of a lithium ion secondary battery to a plug-in hybrid car, it is necessary to decrease its cost, volume and weight while high safety is maintained. For such purposes, positive electrode materials are required to have high capacity and high safety.
JP-A-2008-117611 provides a positive electrode material having excellent float life characteristic and high safety by mixing three kinds of active substances of a lithium-nickel-cobalt manganese composite oxide having a space group R-3m, a lithium-nickel-cobalt composite oxide having a space group R-3m, and a lithium-manganese composite oxide having a space group Fd-3m. Since the lithium-manganese composite oxide of low capacity is contained in an amount of 20 to 35% by weight, this technique cannot obtain a high capacity as required for the application to plug-in hybrid cars and for achieving an object of the present invention.
In JP-A-2005-259703, a positive electrode material formed by mixing a nickel type compound and the cobalt type compound is used to improve the lifetime and safety at normal temperature and high temperature. Since in this technique two kinds of positive electrode active substances having different compositions are mixed, the ratio of at least one of the active substances is 50 mass % or more. This causes significant heat generation in a narrow temperature range at the positive electrode when the temperature of the battery increases due to internal short-circuit. The positive electrode material cannot provide a positive electrode material that causes gradual heat generation in a wide temperature region as in the invention.
JP-A-2004-127694 provides a positive electrode material having good cycle characteristic while high capacity is maintained by using a positive electrode active substance formed by coating the particle of a lithium nickel type composite oxide represented by the general formula LiNi1-zAlzO2 (0.01≦z≦0.1) with a lithium transition metal oxide represented by the general formula: LiNi1-x-yCoxMnyO2. Since in this technique two kinds of positive electrode active substances having different compositions are mixed, the ratio of at least one of the active substances is 50 mass % or more. This causes significant heat generation in a narrow temperature range at the positive electrode when the temperature of the battery increases due to internal short circuit or the like. The positive electrode material cannot provide a positive electrode material that causes gradual heat generation in a wide temperature region as in the invention.
As described above, the prior arts described above have not been able to attain the high capacity and the high safety required for the battery for plug-in hybrid cars simultaneously.